When using diesel fuel as automobile-use fuel, as technology for mixing the diesel fuel and air and uniformly injecting it into the combustion chambers to convert its explosive combustion effect most efficiently to drive power of the engine, the common rail system is used. This is technology for regulating the injection pressure of the fuel by electronic control and also is technology effective for reducing the harmful substances in the exhaust gas. In Europe, this system is made much use of in passenger cars. Due in part to this, the technology for the system has continued to be developed such as with the use of low impurity diesel fuel to obtain higher output, lower fuel consumption, and, further, larger torque.
The common rail system is mainly configured to pump fuel (diesel fuel) from a fuel tank, hold the pumped up fuel in a fuel accumulator called a “common rail” temporarily at a high pressure, transport the fuel under pressure from small sized discharge ports called “orifices” through pipes to the injection nozzles, mix the combustion-use air and fuel inside the nozzles, and uniformly inject the mixtures to the engine combustion chambers.
When discharging the fuel from the injection nozzles, the more uniformly the fuel is injected, the higher the combustion efficiency, and the higher pressure it is injected by, the easier the objective can be realized. That is, designing a fuel injection system injecting fuel at an extremely high pressure is an important technical challenge to be tackled in developing an automobile-use engine with small emission of harmful substances.
However, in current common rail systems, when the fuel is first stored under pressure in the accumulator, in the process leading to the discharge ports, the accumulator itself sometimes cannot withstand the fuel pressure and undergoes fatigue breakage due to the internal pressure.
To solve this problem, it is important to increase the strength of the steel material of the common rail. With this understanding, efforts are being made to deal with this by adjusting the chemical ingredients of the steel material and adjusting the heat treatment conditions in technical development. A common rail system sufficiently reliable up to injected fuel pressures of 120 MPa has already been commercialized.
A common rail for a high pressure over 120 MPa is at the present point of time formed integrally by hot forging, machined into a complicated shape, and further increased in strength by thermal refining, but as the strength of the material becomes higher, the shapeability deteriorates and the processing becomes difficult. Therefore, this method of production invites a large increase in costs. Further, development of technology raising the internal pressure of the common rail more is difficult.
At the present point of time, some common rails for high pressures of up to 150 MPa have been commercialized, but no method of production other than the combination of forging and machining has yet been established. Therefore, the problem of further raising the internal pressure of the common rail remains unsolved.
The inventors fundamentally reevaluated the method of production of a high pressure common rail and took note of the method of dividing each location into parts of simple shapes and mass producing and joining the parts to assemble finished products.
Techniques of forming parts by integral shaping and, when the shapes are complicated, dividing parts which should be produced by die forging, upset forging, casting, or partial cutting into parts of simple shapes for mass production and assembling these by liquid phase diffusion bonding are disclosed in Japanese Patent Publication (A) No. 2002-086279 and Japanese Patent Publication (A) No. 2002-263857.
These techniques utilize the advantage of precision joining technology of liquid phase diffusion bonding and realize parts of complicated shapes by joining, but liquid phase diffusion bonding has the property of advancing limited by the diffusion of the melting point lowering element, so it is necessary to continue to apply stress at the joint faces at a high temperature. The process time, even if just joining, is a relatively long one minute or more and the cost of the joining equipment is high, so these techniques have not spread in industrial use.
Further, Japanese Patent Publication (A) No. 2002-086279 and Japanese Patent Publication (A) No. 2002-263857 do not disclose technology enabling stable precision abutment of the joint faces even with local deformation of the joint faces when the stress applied to the joint faces does not become uniform due to problems with the joint fixtures or shape of the parts or further the processing precision or when the heating is not performed uniformly.
An automobile-use high pressure fuel injection accumulator-distributor is the most important location for obtaining reliability of an internal combustion engine. Due to the nature of the location where it is applied, the joint strength is strictly reflected in design. Therefore, for example, if an incomplete joint happens to occur due to a factor hard to manage in the joining process, that is, a factor such as the above, even for example if making the later inspection technology fail-safe, due to the production costs, the yield will not improve and the cost of the parts will skyrocket. Further, when lowering the precision of the inspection for production, the problem that sufficient reliability as an industrial product cannot be obtained remains unsolved.
Liquid phase diffusion bonding and other surface joining technology enable formation of precision joints, but conversely are sensitive to very slight abnormalities in the groove shapes, that is, parallel degree of the abutting groove faces and the distance between groove faces (also called “groove opening”). Problems remain to be solved in obtaining a joint with a high reliability.